Method and system for characterizing a representative cardiac beat using multiple templates

ABSTRACT

The present invention provides a method and system for characterizing one beat of a patient&#39;s supraventricular rhythm. A plurality of templates is provided and updated using a plurality of qualified beats. Updating occurs by temporally aligning the shock channel waveforms of the template beats using rate channel fiducial points. The template beats are combined by point-by-point addition of the shock channel waveforms. The resultant updated template characterizes one of the patient&#39;s supraventricular conducted cardiac beats.

RELATED PATENT DOCUMENT

This application is a division of U.S. patent application Ser. No.10/105,875 filed on Mar. 25, 2002, to issue on Feb. 27, 2007 as U.S.Pat. No. 7,184,818 which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to implantable medical devicesand, more particularly, to generating, with an implantable medicaldevice, a template characterizing a representative cardiac beat basedupon a minimal number of beats.

BACKGROUND OF THE INVENTION

Rhythmic contractions of a healthy heart are normally controlled by thesinoatrial (SA) node, specialized cells located in the upper rightatrium. The SA node is the normal pacemaker of the heart, typicallyinitiating 60-100 heart beats per minute. When the SA node is pacing theheart normally, the heart is said to be in normal sinus rhythm (NSR).

A heart rhythm which deviates from normal sinus rhythm is an arrhythmia.Arrhythmia is a general term used to describe heart rhythm disturbancesarising from a variety of physical conditions and disease processes.Bradycardia occurs when the heart rhythm is too slow and has a number ofetiological sources including tissue damage due to myocardialinfarction, exposure to toxins, electrolyte disorders, infection, drugeffects, hypoglycemia or hypothyroidism. Bradycardia also may be causedby the sick sinus syndrome, wherein the SA node loses its ability togenerate or transmit an action potential to the atria.

Tachycardia occurs when the rhythm is too fast. The origin of anaberrant tachyarrhythmic impulse may lie in either the atria or theventricles. Supraventricular tachycardia is an atrial arrhythmia and isoften caused by an extra conducting pathway between the atria andventricles. Such a pathway can allow retrograde conduction or electricalimpulses from the ventricles into the atria. The extra pathway incombination with the normal pathway forms a conducting loop thatmodifies the normal heart rhythm. Atrial flutter is caused due toelectrical impulses circulating in the atria. Atrial fibrillation occurswhen the pulses occur in the atria at irregular intervals and usually ata rate of greater than 300 impulses per minute. As a result, pulsesreaching the AV node and thus the ventricles are also irregular, causingirregular contractions of the ventricles at an increased rate.

Ventricular tachycardia occurs when a pulse is initiated in theventricular myocardium with a rhythm more rapid than the normal rhythmof the SA node. Ventricular tachycardia (VT), for example, ischaracterized by a rapid heart beat, 150 to 250 beats per minute andtypically results from damage to the ventricular myocardium from amyocardial infarction. Ventricular tachycardia can quickly degenerateinto ventricular fibrillation (VF). Ventricular fibrillation is acondition denoted by extremely rapid, nonsynchronous contractions of theventricles. The rapid and erratic contractions of the ventricles cannoteffectively pump blood to the body and the condition is fatal unless theheart is returned to sinus rhythm within a few minutes.

Implantable cardioverter/defibrillators (ICDs) have been used as aneffective treatment for patients with serious arrhythmias. ICDs are ableto recognize and treat arrhythmias with a variety of tiered therapies.These tiered therapies include providing anti-tachycardia pacing orcardioversion energy for treating ventricular tachycardia anddefibrillation energy for treating ventricular fibrillation. Toeffectively deliver these treatments, the ICD must first identify thetype of arrhythmia that is occurring, after which appropriate therapy isprovided to the heart. To apply the proper therapy in responding to anepisode of arrhythmia, the ICD may compare sensed cardiac signals to apreviously stored cardiac waveform. The stored cardiac waveform mustaccurately characterize a patient's true supraventricular rhythm (SVR)to properly identify potentially fatal deviations.

Various methods have been used to characterize a patient'ssupraventricular rhythm. Previously described methods often require theacquisition of a relatively large number of heart beat samples toaccurately characterize the patient's SVR. These techniques are notsuitable for use in all cases. When the heart is being paced, forexample, the paced beats are typically discarded from use in templateformation. A large number of supraventricular beats may be difficult toacquire for patients requiring intermittent or constant pacing pulses tobe applied to the heart. Consequently, for these patients, acharacterization of SVR cannot readily be generated or updated byprevious methods.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading thepresent specification, there is a need in the art for a method anddevice that reliably and accurately characterizes a patient's SVRrequiring a minimal number of supraventricular beat samples. Thereexists a further need for such an approach that is adaptive andaccommodates changes in the patient's SVR over time. The presentinvention fulfills these and other needs.

SUMMARY OF THE INVENTION

The present invention is directed to a method and device for generatinga snapshot representative of one beat of a patient's supraventricularrhythm using a minimal number of beats. In accordance with oneembodiment of the present invention, a number of templates are provided.The templates are selectively updated with qualified beats and are usedto characterize the patient's supraventricular rhythm.

In another embodiment of the invention, a patient's supraventricularrhythm is characterized using a first template and a second template. Afirst template and a second template are provided. The first template isupdated with qualified beats correlated to the first template and afirst number of correlated beats associated with the first template iscounted. The second template is updated with qualified beats correlatedto the second template and a second number of correlated beatsassociated with the second template is counted. The first updatedtemplate is stored as a current template if the first number ofcorrelated beats reaches a predetermined count prior to the secondnumber of correlated beats reaching the predetermined count. The secondupdated template is stored as a current template if the second number ofcorrelated beats reaches a predetermined count prior to the first numberof correlated beats reaching the predetermined count. The currenttemplate represents one beat of the patient's supraventricular conductedbeats.

Another embodiment of the invention is directed to a body implantablesystem for implementing SVR characterization. A lead system extends intoa patient's heart and includes one or more electrodes. A detectorsystem, coupled to the lead system, detects rate channel signals andshock channel signals sensed by the one or more electrodes. A controlsystem is coupled to the detector system. The control system provides anumber of templates, selectively updates the templates using a number ofqualified beats, and characterizes the patient's supraventricular rhythmusing the number of templates.

Another embodiment of the invention is directed to a body implantablesystem implementing an SVR characterization method using two templates.The body implantable system includes a lead system that extends into theheart. A detector system, coupled to the lead system, detects ratechannel signals and shock channel signals. A control system, coupled tothe lead system, provides a first and a second template. The controlsystem updates the first template using qualified beats correlated tothe first template and updates the second template using qualified beatscorrelated to the second template. A first number of correlated beatsassociated with the first template is counted and a second number ofcorrelated beats associated with the second template is counted. Thecontrol system stores the first updated template as a current templateif the first number of correlated beats associated with the firsttemplate reaches a predetermined count prior to the second number ofcorrelated beats associated with the second template reaching thepredetermined count. The control system stores the second updatedtemplate as a current template if the second number of correlated beatsassociated with the second template reaches a predetermined count priorto the first number of correlated beats associated with the firsttemplate reaching the predetermined count. The current templaterepresents one beat of the patient's supraventricular conducted beats.

In another embodiment of the invention, a system for characterizing apatient's supraventricular rhythm includes means for providing aplurality of templates, means for selectively updating the plurality oftemplates using a plurality of qualified beats and means forcharacterizing the patient's supraventricular rhythm using a particulartemplate of the plurality of updated templates.

Another embodiment of the invention is a system for characterizing apatient's supraventricular rhythm including means for providing a firsttemplate and a second template, means for detecting qualified beats,means for updating the first template or the second template usingqualified beats correlated to the first template or the second template,means for counting a first number of correlated beats associated withthe first updated template, means for counting a second number ofcorrelated beats associated with the second template, means for storingthe first updated template as a current template if the first number ofcorrelated beats reaches a predetermined count prior to the secondnumber of correlated beats reaching the predetermined count, and meansfor storing the second updated template as a current template if thesecond number of correlated beats reaches a predetermined count prior tothe first number of correlated beats reaching the predetermined count.The current template represents one of the patient's supraventricularconducted beats.

The above summary of the present invention is not intended to describeeach embodiment or every implementation of the present invention.Advantages and attainments, together with a more complete understandingof the invention, will become apparent and appreciated by referring tothe following detailed description and claims taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial view of one embodiment of an implantable medicaldevice with an endocardial lead system extending into atrial andventricular chambers of a heart;

FIG. 2 is a block diagram of a cardiac defibrillator with which SVRcharacterization of the present invention may be implemented;

FIG. 3 is a flowchart of a method of characterizing supraventricularrhythm in accordance with an embodiment of the present invention;

FIG. 4 is a flowchart of a method of characterizing supraventricularrhythm using two templates in accordance with an embodiment of thepresent invention;

FIG. 5 is a more detailed flowchart of a method of characterizingsupraventricular rhythm using two templates in accordance with anembodiment of the present invention;

FIG. 6 is a flowchart of a method of initiating SVR characterization inaccordance with an embodiment of the present invention;

FIG. 7 is a flowchart of a method of forming a template in accordancewith an embodiment of the present invention;

FIG. 8 is a flowchart of a method of acquiring a beat in accordance withan embodiment of the present invention;

FIG. 9 is a flowchart of a method of determining if a beat is aqualified beat in accordance with an embodiment of the presentinvention;

FIG. 10 is a flowchart of a method of determining if a beat iscorrelated to a template in accordance with an embodiment of the presentinvention;

FIG. 11 is a flowchart of a method of updating a template in accordancewith an embodiment of the present invention;

FIGS. 12 and 13 respectively illustrate positive and negative typefiducial points determined from rate channel signals in accordance withan embodiment of the present invention;

FIGS. 14 and 15 show morphological features, including turning point andflat slope features, respectively, for selection of Feature 2 inaccordance with an embodiment of the present invention;

FIGS. 16 and 17 show morphological features, including turning point andflat slope features, respectively, for selection of Feature 4, inaccordance with an embodiment of the present invention; and

FIG. 18 illustrates a method of updating a template by point-by-pointaddition of a number of template beats aligned with respect to a ratechannel fiducial point in accordance with an embodiment of the presentinvention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail below. It is to be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In the following description of the illustrated embodiments, referencesare made to the accompanying drawings which form a part hereof, and inwhich is shown by way of illustration, various embodiments in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized, and structural and functional changes maybe made without departing from the scope of the present invention.

The embodiments of the present system illustrated herein are generallydescribed as being implemented in an implantable cardiac defibrillator(ICD), which may operate in numerous pacing modes known in the art. Thesystems and methods of the present invention may also be implemented inother implantable medical devices that sense cardiac activity, such aspacemakers and cardiac monitors, for example.

In one embodiment, an implantable cardiac defibrillator thatincorporates the systems and methods of the present invention is a dualchamber defibrillator. Various types of single and multiple chamberimplantable cardiac defibrillators are known in the art and mayimplement an SVR characterization methodology of the present invention.

The systems and methods of the present invention may also be implementedin external cardioverter/monitor systems. Also, the present medicalsystem can also be implemented in an implantable atrialcardioverter/defibrillator, which may include numerous pacing modesknown in the art. Furthermore, although the present system is describedin conjunction with an implantable cardiac defibrillator having amicroprocessor-based architecture, it will be understood that theimplantable cardiac defibrillator (or other device) may be implementedin any logic-based architecture, if desired.

Various methods have been used to characterize a patient'ssupraventricular rhythm. One such method is described in commonly ownedU.S. patent application Ser. No. 09/845,987, filed Apr. 30, 2001, andentitled “Normal Cardiac Rhythm Template Generation System And Method,”now U.S. Pat. No. 6,708,058 which is hereby incorporated herein byreference.

The present invention provides a system and method for monitoring apatient's electrocardiogram and producing a characterization of thepatient's normal supraventricular conducted rhythm using fewer beatsthan previous methods. Producing such a characterization may be effectedat any time for a number of different purposes. By way of example, thediagnosis of a patient's cardiac rhythms may be enhanced by comparingQRS complexes of a current cardiac rhythm to a characterization of thepatient's supraventricular cardiac rhythm produced by employment of themethodologies of the present invention. By way of further example, thetitration of drug dosage based on electrocardiographic properties ofsuch a snapshot produced in accordance with the present invention mayalso be enhanced.

The methods of producing an accurate characterization of a patient'ssupraventricular rhythm may be used in combination with an automaticVT/SVT (ventricular tachyarrhythmia/supraventricular tachyarrhythmia)rhythm discrimination technique employed in an implantablecardioverter/defibrillator (ICD). Also, the methodologies of the presentinvention may be used as a component of an automatic Holter analysissystem employed in an implantable pacemaker, for example. These andother applications may be enhanced by employment of the systems andmethods of the present invention.

Referring now to FIG. 1 of the drawings, there is shown one embodimentof a medical device system which includes an implantable cardiacdefibrillator 100 electrically and physically coupled to an intracardiaclead system 102. The intracardiac lead system 102 is implanted in ahuman body with portions of the intracardiac lead system 102 insertedinto a heart 101. The intracardiac lead system 102 is used to detect andanalyze electric cardiac signals produced by the heart 101 and toprovide electrical energy to the heart 101 under certain predeterminedconditions to treat cardiac arrhythmias, including, for example,ventricular fibrillation of the heart 101. In an embodiment in whichonly monitoring of cardiac activity is performed, the intracardiac leadsystem 102 need not provide for the production of electrical energy tostimulate the heart 101.

The intracardiac lead system 102 includes one or more pacing electrodesand one or more intracardiac defibrillation electrodes. In theparticular embodiment shown in FIG. 1, the intracardiac lead system 102includes a ventricular lead system 104 and an atrial lead system 106.The ventricular lead system 104 includes an SVC-coil 116, an RV-coil114, and an RV-tip electrode 112. The RV-coil 114, which is alsoreferred to as an RV-ring electrode, is spaced apart from the RV-tipelectrode 112, which is a pacing electrode. In one embodiment, theventricular lead system 104 is configured as an integrated bipolarpace/shock lead.

The atrial lead system 106 includes an A-tip electrode 152 and an A-ringelectrode 154. In one embodiment, the atrial lead system 106 isconfigured as an atrial J lead.

In this configuration, the intracardiac lead system 102 is positionedwithin the heart 101, with a portion of the atrial lead system 106extending into the right atrium 120 and portions of the ventricular leadsystem 104 extending into the right atrium 120 and right ventricle 118.In particular, the A-tip electrode 152 and A-ring electrode 154 arepositioned at appropriate locations within the right atrium 120. TheRV-tip electrode 112 and RV-coil 114 are positioned at appropriatelocations within the right ventricle 118. The SVC-coil 116 is positionedat an appropriate location within the right atrium chamber 120 of theheart 101 or a major vein leading to the right atrium chamber 120 of theheart 101. The RV-coil 114 and SVC-coil 116 depicted in FIG. 1 aredefibrillation electrodes.

Additional pacing and defibrillation electrodes may also be included inthe intracardiac lead system 102 to allow for various sensing, pacing,and defibrillation capabilities. For example, the intracardiac leadsystem 102 may include endocardial pacing andcardioversion/defibrillation leads (not shown) that are advanced intothe coronary sinus and coronary veins to locate the distal electrode(s)adjacent to the left ventricle or the left atrium. The distal end ofsuch coronary sinus leads is advanced through the superior vena cava,the right atrium, the valve of the coronary sinus, the coronary sinus,and into a coronary vein communicating with the coronary sinus, such asthe great vein. Other intracardiac lead and electrode arrangements andconfigurations known in the art are also possible and considered to bewithin the scope of the present system.

The ventricular and atrial lead systems 104, 106 include conductors forcommunicating sense, pacing, and defibrillation signals between thecardiac defibrillator 100 and the electrodes and coils of the leadsystems 104, 106. As is shown in FIG. 1, the ventricular lead system 104includes a conductor 108 for transmitting sense and pacing signalsbetween the RV-tip electrode 112 and an RV-tip terminal 202 within thecardiac defibrillator 100. A conductor 110 of the ventricular leadsystem 104 transmits sense signals between the RV-coil or ring electrode114 and an RV-coil terminal 204 within the cardiac defibrillator 100.The ventricular lead system 104 also includes conductor 122 fortransmitting sense and defibrillation signals between terminal 206 ofthe cardiac defibrillator 100 and the SVC-coil 116. The atrial leadsystem 106 includes conductors 132, 134 for transmitting sense andpacing signals between terminals 212, 210 of the cardiac defibrillator100 and A-tip and A-ring electrodes 152 and 154, respectively.

Referring now to FIG. 2, there is shown an embodiment of a cardiacdefibrillator 100 suitable for implementing a supraventricular rhythmtemplate generation methodology of the present invention. FIG. 2 shows acardiac defibrillator divided into functional blocks. It is understoodby those skilled in the art that there exist many possibleconfigurations in which these functional blocks can be arranged. Theexample depicted in FIG. 2 is one possible functional arrangement. Thecardiac defibrillator 100 includes cardiac defibrillator circuitry 203for receiving cardiac signals from a heart 101 (not shown in FIG. 2) anddelivering electrical energy to the heart. The cardiac defibrillator 100includes terminals 202, 204, 206, 209, 210, and 212 for connecting tothe electrodes and coils of the intracardiac lead system as previouslydiscussed.

In one embodiment, the cardiac defibrillator circuitry 203 of thecardiac defibrillator 100 is encased and hermetically sealed in ahousing 130 suitable for implanting in a human body as is known in theart. Power to the cardiac defibrillator 100 is supplied by anelectrochemical battery 256 that is housed within the cardiacdefibrillator 100. A connector block (not shown) is additionallyattached to the housing 130 of the cardiac defibrillator 100 to allowfor the physical and electrical attachment of the intracardiac leadsystem conductors to the cardiac defibrillator 100 and the encasedcardiac defibrillator circuitry 203.

In one embodiment, the cardiac defibrillator circuitry 203 of thecardiac defibrillator 100 is a programmable microprocessor-based system,with a control system 201 and a memory circuit 257. The memory circuit257 stores parameters for various pacing, defibrillation, and sensingmodes and stores data indicative of cardiac signals received by othercomponents of the cardiac defibrillator circuitry 203. The controlsystem 201 and memory circuit 257 cooperate with other components of thecardiac defibrillator circuitry 203 to perform operations involving thegeneration of a template representing a snapshot of one beat of apatient's supraventricular rhythm according to the principles of thepresent invention, in addition to other sensing, pacing anddefibrillation functions. A memory 213 is also provided for storinghistorical EGM and therapy data, which may be used on-board for variouspurposes and transmitted to an external programmer unit 228 as needed ordesired.

Telemetry circuitry 224 is additionally coupled to the cardiacdefibrillator circuitry 203 to allow the cardiac defibrillator 100 tocommunicate with an external programmer unit 228. In one embodiment, thetelemetry circuitry 224 and the programmer unit 228 use a wire loopantenna and a radio frequency telemetric link, as is known in the art,to receive and transmit signals and data between the programmer unit 228and cardiac defibrillator circuitry 203. In this manner, programmingcommands and instructions are transferred to the control system 201 ofthe cardiac defibrillator 100 from the programmer unit 228 during andafter implant, and stored cardiac data pertaining to sensed arrhythmicepisodes within the heart 101, template information, and subsequenttherapy or therapies applied to correct the sensed arrhythmic event aretransferred to the programmer unit 228 from the cardiac defibrillator100, for example.

Cardiac signals sensed through use of the RV-tip electrode 112 arenear-field signals or rate channel signals as are known in the art. Moreparticularly, a rate channel signal is detected as a voltage developedbetween the RV-tip electrode 112 and the RV-coil 114. Rate channelsignals developed between the RV-tip electrode 112 and the RV-coil 114are referred to herein as rate channel signals or signals measured fromthe rate channel.

Cardiac signals sensed through use of one or both of the defibrillationcoils or electrodes 114, 116 are far-field signals, also referred to asmorphology or shock channel signals, as are known in the art. Moreparticularly, a shock channel signal is detected as a voltage developedbetween the RV-coil 114 and the SVC-coil 116 or the can electrode 209. Ashock channel signal may also be detected as a voltage developed betweenthe RV-coil 114 and the SVC-coil 116 coupled to the can electrode 209.Shock channel signals developed using appropriate combinations of theRV-coil, SVC-coil, and can electrodes 114, 116 and 209 are sensed andamplified by a shock EGM amplifier 238 located in the detector system260. The output of the EGM amplifier 238 is coupled to the controlsystem 201 via the signal processor and A/D converter 222.

In the embodiment of the cardiac defibrillator 100 depicted in FIG. 2,RV-tip and RV-coil electrodes 112, 114 are shown coupled to a V senseamplifier 230 located within the detector system 260. Rate channelsignals received by the V-sense amplifier 230 are communicated to thesignal processor and A/D converter 222. The detector system serves tosense and amplify the rate channel signals. The signal processor and A/Dconverter 222 converts the R-wave signals from analog to digital formand communicates the signals to the control system 201.

A-tip and A-ring electrodes 152, 154 are shown coupled to an A-senseamplifier 220 located within the detector system 260. Atrial sensesignals received by the A-sense amplifier 220 in the detector system 260are communicated to an A/D converter 222. The A-sense amplifier servesto sense and amplify the A-wave signals. The A/D converter 222 convertsthe sensed signals from analog to digital form and communicates thesignals to the control system 201.

The pacemaker 240 communicates pacing signals to the RV-tip and A-tipelectrodes 112 and 152 according to a preestablished pacing regimenunder appropriate conditions. Blanking circuitry (not shown) is employedin a known manner when a ventricular or atrial pacing pulse isdelivered, such that the ventricular channel, atrial channel, and shockchannel are properly blanked at the appropriate time and for theappropriate duration.

The cardiac defibrillator 100 depicted in FIG. 1 is well-suited forimplementing a SVR characterization methodology according to theprinciples of the present invention. In the embodiment shown in FIG. 2,the SVR characterization processes of the present invention are carriedout by the template generator 250. The shock channel and rate channelsignals used for SVR characterization and related template operationsare provided by the shock EGM amplifier 238 and the V-sense amplifier230, respectively. It is understood that the required shock and ratechannel signals may be developed and processed by components other thanthose depicted in FIG. 2 for system architectures that differ from thesystem architectures described herein.

FIG. 3 is a flowchart illustrating various processes for characterizinga patient's supraventricular rhythm according to an embodiment of thepresent invention. Characterization of a patient's supraventricularrhythm is accomplished through multiple stages, including, for example,iterative steps. The SVR characterization may be performed or updatedperiodically as needed or desired. According to the embodimentillustrated in the flowchart of FIG. 3, and in broad and general terms,upon commencement of SVR characterization, a number of templates isprovided 310. A qualified beat is detected 320 and used to update 330 atemplate correlated to the beat. The process continues the loop 320 to340 until the template update is complete. If the template updateprocess is complete 340, the templates are used to characterize 350 thepatient's supraventricular rhythm. The process depicted in FIG. 3 may beterminated for various reasons as described hereinbelow.

Turning now to FIG. 4, various processes are illustrated forcharacterization of one beat of a patient's supraventricular rhythmaccording to another embodiment of the present invention. In thisexemplary embodiment, two templates are used to characterize thesupraventricular rhythm of a patient. Upon initiation of SVRcharacterization, a first template is provided 401 and a second templateis provided 405. A qualified beat is detected 410. If the qualified beatis correlated to the first template 415, the first template is updated425. If the qualified beat is uncorrelated to the first template, but iscorrelated to the second template 420, the second template is updated430. If the beat is not correlated to either template, neither templateis updated. The first and second templates continue to be updated 425,430 by qualified beats in this manner until one of the templates isupdated with a sufficient number of qualified beats.

If the number of beats correlated to the first template is equal to apredetermined count 435, the first template is saved 440 as arepresentation of the patient's supraventricular rhythm and SVRcharacterization is complete 455. If the number of beats correlated tothe first template is less than a predetermined count 435 and the numberof beats correlated to the second template is equal to a predeterminedcount 445, the second template is saved 450 as a representation of thepatient's supraventricular rhythm and SVR characterization is complete455.

FIG. 5 is a more detailed illustration of various steps associated withSVR characterization using two templates in accordance with anembodiment of the present invention. According to this embodiment,following commencement of SVR characterization 501, if a stored templateexists 502, the stored template may be used as a first template 503 anda first template counter is set equal to one 507. Alternatively, aqualified beat is acquired 504, 505 and is used as the first template506, and the first template counter is set equal to one 507.

According to the method of the exemplary embodiment, beats are acquired508 until a qualified beat is detected 509. If the qualified beatcorrelates to the first template 510, the first template is updated 514and the first template counter is incremented by one 515. If thequalified beat does not correlate to the first template 510, and thesecond template counter is zero 511, the qualified beat is used to formthe second template 516 and the second template counter is set equal toone 517. If the second template has already been formed, and thequalified beat is correlated to the second template 512, the qualifiedbeat is used to update the second template 518 and the second templatecounter is incremented by one 519.

If a qualified beat is not correlated to either template 510, 512, atemplate with a counter of one 513, 521 may be replaced 520, 523 by thequalified beat. If the second template counter has a value of one 513,then the second template is replaced 520 by the qualified beat. If thesecond template counter has a value greater than one 513 and the firsttemplate counter equals one 521, and the first template was not providedby retrieving a stored template from memory 522, then the first templatemay be replaced by the qualified beat 523. If the first template wasprovided by retrieving a stored template from memory 522, the controlsystem may determine that the first template should be given a higherweight. In this situation, the first template may not be replaced by thequalified beat.

The loop beginning at block 508 is repeated until a predetermined numberof beats is detected 524 or until one of the template counters isincremented to a value equal to a predetermined count, in this example,six counts 525, 526. If the template 1 counter is incremented to a valueequal to six counts, 525, template 1 is stored as the characterizationof the patient's supraventricular rhythm 527 and SVR characterization iscomplete 530. If the template 1 counter is not equal to six counts andthe template 2 counter is equal to six counts 526, template 2 is storedas the characterization of the patient's supraventricular rhythm 528 andSVR characterization is complete 530. If neither template counter isincremented to a value equal to six counts before the predeterminednumber of beats is detected 524, the SVR characterization fails 529 andthe characterization process is terminated 531.

FIG. 6 is a more detailed illustration of various processes 600associated with initiating SVR characterization in accordance with anembodiment of the present invention. RR intervals are developed from thesensed rate channel signals. An RR interval is measured as an intervalbetween Vs to Vs, Vs to Vp, Vp to Vs, or Vp to Vp events, where Vs isthe ventricular sensed event detection time and Vp is the ventricularpace pulse delivery time.

The initial RR average (RRavg) may be calculated as the average of thefirst four RR intervals 610. In one embodiment, the RRavg is calculatedas a running average as is characterized in Equation 1 below:RRavg(I)=0.875*RRavg(I−1)+0.125*RR(I)  [1]Equation 1 above represents one method for determining the RR average.Other methods are known in the art that can be used successfully toobtain the RR average.

A beat is classified as a “regular” beat when an RR interval is largerthan 87.5% and less than 150% of the RRavg. The first qualified beat isavailable only after an initial RRavg value is calculated.

Heart rate is classified as “regular” if at least 40% of the beats areregular. According to one approach, heart rate regularity is checked. Ifthe rate is not regular, the SVR characterization is suspended until thenext scheduled SVR characterization time. By this method, the RRavg andrate regularity are continuously calculated for every beat during theSVR characterization procedure. If the rate becomes too high, or therate becomes irregular at any stage of the SVR characterizationprocedure, the SVR characterization is suspended immediately.

Initiating SVR characterization in accordance with an embodiment of thepresent invention includes performing shock channel automatic gaincontrol (AGC) adjustment 620. Shock channel AGC is performed in thisembodiment by measuring the peak value in four regular beats andadjusting the shock channel gain such that the averaged peak value is50% of the maximum A/D converter value. After the SVR characterizationprocedure is completed, shock channel AGC is readjusted until the nextupdate.

FIG. 7 provides a more detailed illustration of the processes associatedwith forming a template in accordance with an embodiment of the presentinvention. In general terms, a template is a combination of one or morebeats, wherein the combination of beats may represent one beat of thepatient's supraventricular rhythm. According to this embodiment, atemplate is formed by determining the fiducial point type and thefiducial point of the rate channel signal of the initial template beat,and identifying the value and location of features of the initial shockchannel waveform relative to the rate channel fiducial point.

A fiducial point represents a peak value of the rate channel signal. Afiducial point type is either positive (Pos), associated with a positivepeak, or negative (Neg), associated with a negative peak. When atemplate is formed, the positive peak (Pos) or the negative peak (Neg)of the rate channel signal used to form the template determines thefiducial point type of the template. FIGS. 12 and 13 depict positive andnegative fiducial points, respectively. The Pos and Neg peaks aremeasured as absolute values. The fiducial point type is determined byEquation 2 as follows:If Pos>0.9*Neg, the fiducial point type is positiveIf Pos≦0.9*Neg, the fiducial point type is negative  [2]

If a stored template exists, the fiducial point type of the storedtemplate is used as the fiducial point type of the template. If nostored template exists, the fiducial point type of the first beat usedto form the template is used as the fiducial point type for thetemplate.

Returning to FIG. 7, when a template is formed 700, a fiducial pointtype is determined 710 as set forth in the above paragraph, and one ormore features of the shock channel waveform are identified 720. In oneembodiment of the invention, and with reference to FIGS. 14 and 15, fivefeatures are initially identified for the shock channel template,followed by three additional features determined at midpoints betweencertain ones of the five initially selected features.

Feature 3 is selected as the absolute maximum peak in a feature windowdefined by 31 samples centered at the fiducial point. If the positivepeak amplitude is equal to the negative peak amplitude, the positivepeak is selected as Feature 3.

Feature 2 is found by searching backward from Feature 3 until a point isreached that meets the following conditions: 1) the search is limited to10 samples. If no point satisfies the following conditions, then the10th sample becomes Feature 2; 2) the amplitude is less than 25% of themaximum peak; 3) a turning point is found or the slope is flat, and 4)Feature 2 is at least 4 samples away from Feature 3.

By way of example, let Q(I) represent the current sample. A turningpoint is found if:Q(I−1)≧Q(I) and Q(I)<Q(I+1) for a positive Feature 3Q(I−1)≦Q(I) and Q(I)>Q(I+1) for a negative Feature 3  [3]

As is shown in FIG. 14, Q(I) is selected as Feature 2. As such, Feature2 is selected as a turning point.

The slope is considered flat, as shown in FIG. 15, ifabs(Q(I+1)−Q(I−1))<4 and abs(Q(I+1)−Q(I−2))<4, in the case when the A/Dconverter maximum value is 128. In the illustrative depiction of FIG.15, Q(I) is selected as Feature 2. As such, Feature 2 is selected as aflat slope point.

Feature 4 is found by searching forward starting from Feature 3 until apoint is reached that meets the following conditions: 1) the search islimited to 16 samples. If no point satisfies the following conditions,then the 16th sample becomes Feature 4; 2) the amplitude is less than25% of the maximum peak; and 3) a turning point is found or the slope isflat.

By way of example, let Q(I) represent the current sample. A turningpoint is found ifQ(I+1)≧Q(I) and Q(I)<Q(I−1) for a positive Feature 3Q(I+1)>Q(I) and Q(I)>Q(I−1) for a negative Feature 3  [4]

Q(I) is selected as Feature 4, as is shown in FIG. 16.

The slope is flat, as shown in FIG. 17, if abs(Q(I−1)−Q(I+1))<4 andabs(Q(I−1)−Q(I+2))<4. In this case, Q(I) is selected as Feature 4.

Feature 1 is selected as the seventeenth sample from the beginning ofthe detection window. Feature 5 is selected as the last sample of thedetection window. Three additional features are selected at the midpointof Features 1 and 2, the midpoint of Features 2 and 3, and the midpointof Features 3 and 4, respectively. If a midpoint falls between twosample points, the leftmost (earlier in time) point is selected. Thus,according to this embodiment, eight feature values (e.g., amplitudes)and their associated locations with respect to the fiducial point andthe corresponding fiducial point type are saved for SVRcharacterization.

FIG. 8 provides a more detailed illustration of the process of acquiringa beat in accordance with an embodiment of the present invention. Asdiscussed above, cardiac signals sensed through use of the RV-tipelectrode are rate channel signals. Cardiac signals sensed through useof one or both of the defibrillation coils or electrodes are shockchannel signals. When a beat is acquired, the rate channel signal issensed 810 and the shock channel signal is sensed 820.

FIG. 9 illustrates a method of determining if a beat is a qualified beatin accordance with the present invention. Four criteria must be presentfor a beat to be considered a qualified beat suitable for forming orupdating a template. First, the beat and the preceding beat must beintrinsic beats 905. Second, the preceding beat must have a V-V intervallarger than approximately 500 ms and the beat must be regular 910.Third, the shock channel R-wave amplitude must be larger thanapproximately 25% of the maximum value of the A/D converter and must notbe saturated 920. Finally, the rate channel R-wave amplitude must belarger than approximately 50% of the maximum value of the A/D converterand must not saturate the A/D converter at more than one consecutivesample point 930. If all four of these conditions are detected, then thebeat is a qualified beat suitable for characterizing the patient'ssupraventricular rhythm.

Turning now to FIG. 10, a more detailed illustration of various stepsassociated with determining if a qualified beat is correlated to atemplate in accordance with an embodiment of the present invention isprovided. The method illustrated in FIG. 10 may be used to determine ifa beat is correlated to the first or the second template. According tothis method, the fiducial point is determined from the rate channelsignal of the qualified beat 1005. The shock channel waveforms of thetemplate and the qualified beat are aligned using the fiducial points ofthe template and the qualified beat 1010. A number of features of thequalified beat are determined at the locations relative to the fiducialpoint previously determined for the template 1015. The template and thequalified beat are compared by calculating a feature correlationcoefficient (FCC) 1020. In one particular embodiment, Equation 5,provided below, is used to compute the FCC between the template featuresand the beat features. $\begin{matrix}{{FCC} = \frac{( {{N{\sum\limits_{i = 1}^{N}{X_{i}Y_{i}}}} - {( {\sum\limits_{i = 1}^{N}X_{i}} )( {\sum\limits_{i = 1}^{N}Y_{i}} )}} )^{2}}{( {{N{\sum\limits_{i = 1}^{N}X_{i}^{2}}} - ( {\sum\limits_{i = 1}^{N}X_{i}} )^{2}} )( {{N{\sum\limits_{i = 1}^{N}Y_{i}^{2}}} - ( {\sum\limits_{i = 1}^{N}Y_{i}} )^{2}} )}} & \lbrack 5\rbrack\end{matrix}$where, Xi represents template N features and Yi represents beat Nfeatures, and N=8 in this illustrative example. The sign of thenumerator term is checked before squaring. If the numerator is negative,the beat is uncorrelated, and the remainder of the computation need notbe performed.

If the FCC is greater than a predetermined value, as tested at block1025, for example 0.95, then the qualified beat is correlated 1035 tothe template. If the FCC is less than or equal to the predeterminedvalue, then the qualified beat is uncorrelated 1030 to the template.

FIG. 11 illustrates a method of updating a template with a qualifiedbeat correlated to the template in accordance with an embodiment of theinvention. The template may be updated by point-by-point addition 1105of the qualified beat to the template, the updated template being thesum of the addition. The template counter is incremented 1110. Thefeatures of the updated template are identified 1120.

When a qualified beat is correlated to a template, it represents atemplate beat and is used to update the template. After temporalalignment using the rate channel fiducial points, the shock channelwaveforms of the template and the qualified beat may be combined bypoint by point addition. For example, the template may be characterizedby the following equation: $\begin{matrix}{{{Template}( {i,j} )} = {\sum\limits_{i = 1}^{N}{{Template}\quad{{Beat}( {i,j} )}}}} & \lbrack 6\rbrack\end{matrix}$where the term Template Beat(i,j) is the j^(th) sample of the i^(th)template beat of the template, and the initial template is designated asthe template beat for i=1.

The procedure of template updating is illustrated diagrammatically inFIG. 18. The shock channel waveform of the initial template representingthe first template beat 1810 is temporally aligned to the shock channelwaveform of the first qualified beat correlated to the template 1820using the rate channel fiducial points. The template beats are combinedby point by point addition of j samples of the two beats 1830. The sumof the addition becomes the updated template 1840. The updated template1840 is added in the same manner to the next qualified beat 1850 whichis correlated to the updated template. This process continues until allthe qualified beats have been combined 1890. In an example of thismethod discussed previously, the number of qualified beats combined issix. The updated template 1890 may be normalized by dividing by samplesof the updated template by one plus the number of qualified, correlatedbeats used to form the updated template, in this example, six. Thenormalized, updated template is stored as a characterization of thepatient's supraventricular rhythm 1895.

Characterization of a patient's supraventricular rhythm in accordancewith the principles of the present invention provides for severaladvantages. For example, the method of template generation of thepresent invention requires only beat-by-beat analysis and is efficientin memory usage making it well-suited for use in implantable devices,such as in implantable cardioverter/defibrillator devices. Further,template generation is possible using a relatively small number of beatsas compared to previous methods, making the template generation methodof the present invention particularly useful when the patient's heart isbeing intermittently or constantly paced.

Systems and methods of the present invention have been described in theabove discussion using illustrative examples wherein two templates areused to characterize a patient's supraventricular rhythm. The systemsand methods of the invention, however, are not limited to use of twotemplates and may be extended to any number of templates. In some cases,particularly where the morphology of successive cardiac beats variessignificantly, it may be beneficial to provide three or more templatesfor SVR characterization. Extending the example described above to Ntemplates, a detected qualified beat may be compared to each of Ntemplates to determine correlation. When a qualified beat is correlatedto a specific template, such as template x for example, template x isupdated with the qualified beat and the template x counter is updated.When one of the N template counters is equal to a predetermined numberof beats, the template corresponding to that template counter is storedas a characterization of the patient's supraventricular rhythm. In thisway, any number of additional templates and additional template countersmay be readily incorporated into the algorithm as desired.

Various modifications and additions can be made to the preferredembodiments discussed hereinabove without departing from the scope ofthe present invention. Accordingly, the scope of the present inventionshould not be limited by the particular embodiments described above, butshould be defined only by the claims set forth below and equivalentsthereof.

1. A method of characterizing a patient's cardiac rhythm in animplantable device, comprising: detecting a cardiac beat; extractingfeature timing information from the cardiac beat and each template of aplurality of templates; temporally aligning the cardiac beat with atleast one template of the plurality of templates based on the featuretiming information; selectively updating the plurality of templatesusing the aligned cardiac beat; and identifying a particular template ofthe plurality of templates as representative of the cardiac rhythm. 2.The method of claim 1, wherein detecting the cardiac beat comprisesdetecting a two channel cardiac beat signal.
 3. The method of claim 1,wherein aligning the cardiac beat signal and the at least one templatecomprises: extracting a fiducial point of the cardiac beat; extracting afidicial point of the at least one template; and temporally aligning thecardiac beat and the template based on the extracted fiducial points. 4.The method of claim 3, wherein selectively updating the plurality oftemplates comprises: extracting features from the aligned cardiac beathaving predetermined temporal relationships to the fiducial point of theat least one template; and updating the at least one template using thecardiac beat if the extracted features are similar to correspondingtemplate features.
 5. The method of claim 1, wherein selectivelyupdating the plurality of templates using the aligned cardiac beatcomprises: calculating a feature correlation coefficient for the cardiacbeat and a template of the plurality of templates; and updating thetemplate if the feature correlation coefficient is greater than apredetermined value.
 6. The method of claim 1, wherein identifying theparticular template comprises identifying the particular template asrepresentative of the cardiac rhythm based on a number of times theparticular template is updated.
 7. The method of claim 1, whereinidentifying the particular template comprises: for each template,incrementing a counter if the template is updated; and identifying theparticular template as representative of the cardiac rhythm if theparticular template has a highest counter value.
 8. The method of claim1, wherein identifying the particular template comprises: for eachtemplate, incrementing a counter if the template is updated; andidentifying the particular template a representative of the cardiacrhythm if the counter for the particular template reaches apredetermined value before the counters for other templates reach thepredetermined value.
 9. The method of claim 1, further comprising:retrieving at least one template of the plurality of templates frommemory; and generating at least one template of the plurality oftemplates using a previous cardiac beat.
 10. A body implantable systemfor characterizing cardiac rhythms, comprising: a sensing systemcomprising electrodes electrically coupled to a heart, the sensingsystem configured to detect a cardiac beat; and a control system coupledto the sensing system, the control system configured to temporally alignthe cardiac beat with at least one template of a plurality of templatesbased on feature timing information extracted from the cardiac beat andthe templates, selectively update the plurality of templates using thealigned cardiac beat, and identify a particular template of theplurality of templates as representative of the cardiac rhythm.
 11. Thesystem of claim 10, further comprising a memory, wherein the controlsystem retrieves at least one template of the plurality of templatesfrom the memory.
 12. The system of claim 10, wherein the control systemis configured to generate a new template using the cardiac beat if notemplates are similar to the cardiac beat.
 13. The system of claim 10,wherein: the feature timing information extracted from the cardiac beatcomprises a fiducial point of the cardiac beat; and the feature timinginformation extracted from the templates comprises fidicual points ofthe templates.
 14. The system of claim 10, wherein: the feature timinginformation extracted from the cardiac beat comprises a peak value ofthe cardiac beat; and the feature timing information extracted from thetemplates comprises peak values of the templates.
 15. The system ofclaim 10, further comprising counters associated with each template,wherein the counters are incremented each time a template is updated.16. The system of claim 15, wherein the control system is configured toidentify the particular template as representative of the cardiac rhythmbased on a value of a counter associated with the particular template.17. The system of claim 15, wherein the control system is configured toidentify the particular template as representative of the cardiac rhythmif the particular template has a counter value higher than othertemplates.
 18. The system of claim 15, wherein the control system isconfigured to identify the particular template as representative of thecardiac rhythm if a counter associated with the particular templatereaches a predetermined value before counters associated with othertemplates.
 19. An implantable cardiac system, comprising: means foraligning a cardiac beat with each template of a plurality of templatesbased on feature timing information extracted from the cardiac beat andthe templates; means for selectively updating one or more templates ofthe plurality of templates using the aligned cardiac beat; and means foridentifying a particular template of the selectively updated templatesas representative of the cardiac rhythm.
 20. The system of claim 19,further comprising: means for retrieving at least one template of theplurality of templates from memory; and means for generating at leastone template of the plurality of templates using a previous cardiacbeat.